Alkali-free accelerators for sprayed concrete

The application of wet sprayed concrete requires the accelerating component to achieve setting of the concrete and strength development. Common accelerators are highly alkaline and contain substantial amounts of alkali. Corrosive aerosols produced during addition of the accelerator at the nozzle can cause serious problems to skin and eyes of workers. Other disadvantages of the traditional accelerators are damage due to alkali-silicate reaction and a reduction of the mechanical strength of up to 50% over time. A goal of the worldwide operating R&D division of the Underground Construction Group of MBT was the development of an alkali-free accelerator to overcome some problems of sprayed concrete applications and contamination by leaching water. This paper will show some results with these types of accelerators from laboratory tests and practical applications at interesting job sites world-wide.